The invention relates to an apparatus for removing jitter in a color television signal and, more particularly, an apparatus in which a time-axial variation component, called jitter, in a color television signal recorded by video tape recorder (VTR), for example, is detected and corrected.
Color television of the National Television System Committee (NTSC) is compatible with monochrome television. For this, this type color television broadcasting uses the same 6 MHz broadcast channels as monochrome transmission. In this color television system, color video signals, or R, G, B signals picked up by color television cameras are combined to convert the picture information into the luminance signal and chrominance signal. The luminance signal corresponds to the video signal in the monochrome television and is transmitted through the same 4.2 MHz channel as monochrome television. The chrominance signal includes only the color information of hue and chromaticity of a color. The chrominance signal is divided, in accordance with the visual perception of human, into the I signal (the bandwidth 1.5 MHz) and the Q signal (the bandwidth 0.5 MHz). These signals are superposed on a subcarrier 3.58 MHz and the superposed one is transmitted through the 6 MHz channel. The color television signal is composed of color video signals, horizontal and vertical synchronizing signals, and a chrominance subcarrier 3.58 MHz. This chrominance subcarrier is called a color burst 8 to 12 cycles superposed on the back porch of each horizontal synchronizing pulse after horizontal blanking period for demodulation of the television signal. The color burst synchronizes the phase of a subcarrier oscillator for demodulation.
In such a type color television signal, the hue depends largely on the phases of the color video signals. Accordingly, in order to secure a excellent color picture quality, the transmission system must have a good frequency characteristic and the circuit system must also permit correct and stable phase of the signal circuiting therein.
As described above, the color video signal includes the luminance signal for carrying the information contents of black and white picture and the chrominance signal resulting from the quadrature phase modulation of the chrominance subcarrier 3.58 MHz by color signals. There are some methods to record such a composite color video signal by a VTR. One of them is that, as in the monochrome television, the color video signal is frequency-modulated and the frequency-modulated one is directly recorded on a recording medium, i.e. a video tape. In this case, the frequency of the carrier wave must be high. However, it is difficult to manufacture record/play-back heads to record and play back such a high frequency signal. Therefore, the 3.58 MHz of the subcarrier is converted into a lower frequency in the frequency range of the VTR. The VTR employing such a method in general uses a rotary head. This type VTR suffers from jitter due to uneven rotation of the rotary head, extension or contraction of a video tape, speed variation in the video tape running, etc. The jitter frequency-modulates the chrominance subcarrier so that the 3.58 frequency band is instable with phase change. This leads to change of the hue of the play-back picture.
The jitter in recording and playing back the color video signal by the VTR has been corrected in the following manner. The luminance signal is frequency-modulated and the chrominance signal is converted into a signal with low frequency by using the subcarrier 3.58 MHz, in the record/play-back operation of the color video signal. The play-back color video signal including the jitter is divided into the luminance signal and the chrominance signal. The jitter included in the luminance signal is eliminated through a delay line. The jitter included in the chrominance signal is eliminated by a phase control circuit. This method is discussed in Japanese Patent Gazette No. 14852/75, for example.
In this correction method, the jitter is separately corrected in the luminance and chrominance signal processing circuits. Therefore, the jitter in the chrominance or luminance signal per se is eliminated but a phase difference is produced between them. This results in color edging of the play-back picture.
To overcome this disadvantage, another jitter correction method was proposed in which the horizontal synchronizing signal of a color television signal is phase-compared with the reference oscillating signal generated by a flywheel oscillator in the recording/play-back system and the result of the comparison is used for the jitter correction. As a video tape recorder employing this method, an automatic video tape recorder AVR-2 is enumerated which is manufactured by TOSHIBA AMPEX CO., Ltd. However, this type apparatus is defective in that the construction is complex and the cost is high, and that a high precision of the jitter detection can not be expected. Therefore, the play-back picture suffers color shading although it is slight. To sufficiently correct the residual jitter, the flywheel oscillator must be of very high precision. However, it is practically very difficult to manufacture such a high precision oscillator. Additionally, when skew distortion takes place, it is impossible to detect the jitter. None of the countermeasure has been taken for this problem.